Field of the Invention
The invention relates to a method for manufacturing a hot press-hardened component, to a use of a steel product for manufacturing a hot press-hardened component and to a hot press-hardened component.
Description of the Related Art
To meet the current demand in modern vehicle body construction for less weight with at the same time maximum strength and protective effect, nowadays hot press-formed components made of high-strength steels are used in those areas of the vehicle body which in the event of a crash can be subjected to particularly heavy stresses.
In hot press-hardening, steel blanks which are separated from cold-rolled or hot-rolled steel strip are heated at a deformation temperature which is usually above the austenitisation temperature of the respective steel and are placed in the heated state into the die of a forming press. In the course of the forming subsequently carried out, the sheet blank or the component formed from it undergoes rapid cooling through contact with the cool die. The cooling rates are set in such a way that a martensitic structure develops in the component. Here, it can be sufficient for the component to be cooled by contact with the die alone without active cooling. However, rapid cooling can also be supported by the die itself being actively cooled.
As reported in the article “The potential for vehicle body lightweight construction” which appeared in the ThyssenKrupp Automotive AG trade show journal for the 61st Frankfurt International Motor Show, 15-25 Sep. 2005, hot press-hardening is in practice particularly used for manufacturing high-strength vehicle body components made of boron-alloyed steels. A typical example for such a steel is known under the designation “22MnB5” and can be found in the Key to Steel 2004 under the material number 1.5528.
The advantages of the known MnB steels are, however, in practice confronted with the disadvantage that steels with a high manganese content are too unstable against wet corrosion and can only be passivated with difficulty. This strong susceptibility to corrosion compared to more lowly alloyed steels with the action of increased chloride ion concentrations, which although it is limited locally is intensive, makes the use of steels belonging to the high-alloyed steel sheet material group difficult specifically in vehicle body construction. In addition, steels with a high manganese content are susceptible to surface corrosion, as a result of which the range for their use is also restricted.
Therefore, it has been proposed that steel flat products which are produced from steels with a high manganese content are also provided with a metallic coating, in a manner which is known per se, which protects the steel against corrosive attack. At the same time, however, the problem arose that such steel flat products can only be poorly wetted and consequently the adhesion to the steel substrate required from the coating during cold forming is not adequate.
A large number of proposals have been made for providing steel flat products produced from a steel with a high manganese content with a coating which protects against corrosion and which meets the requirements demanded in practice (DE 10 2005 008 410 B3, WO 2006/042931 A1, WO 2006/042930, DE 10 2006 039 307 B3 and many others). The common link between these proposals is that the steel flat product, which is to be coated in each case, has to be annealed in an annealing step, which is elaborate and difficult to control in terms of the technical process due to the conditions to be followed, so that it can subsequently be provided with the corrosion protection coating in an appropriate coating process. Furthermore, it has been shown that the coating of the steel flat products results in abrasion particularly on the rollers of the furnaces. As a result of this abrasive wear, a premature replacement or other maintenance measures are required, which are associated with long downtimes.